Story highlights

Testimony focuses on shooting death of MIT officer, 26

Sean Collier killed three days after Boston bombings

Cyclist IDs Tsarnaev, puts him at scene of Collier killing

Boston (CNN)It was 9:35 on a slow Thursday night in April 2013 and Massachusetts Institute of Technology Police Chief John DiFava was about to call it quits. On his way out, he saw one of his rookie swing-shift officers, Sean Collier, sitting in his cruiser. He stopped to say goodnight.

"I chatted with him for a few minutes. I told him to be safe and I left," the chief told a crowded courtroom on Wednesday. He estimated the conversation lasted three, maybe four minutes.

"Did you ever see Sean Collier alive again after that?" Assistant U.S. Attorney William Weinreb asked.

"I did not."

Less than an hour later, Collier lay bleeding in his patrol car after being ambushed and shot in the head. His car door was open, and his foot was lodged between the gas and brake pedals.

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DiFava and other officers, assisted by surveillance videos, 911 callers and a lone bicyclist who happened to be passing by, recounted Collier's last moments in the death penalty trial of admitted Boston Marathon bomber Dzhokhar Tsarnaev.

The bicyclist, MIT mathematics Ph.D. candidate Nathan Harman, pointed to Tsarnaev in court and identified him as the man with "a big nose," who he saw leaning into Collier's squad car. He said Tsarnaev appeared to be alone.

Tsarnaev, who was 19 at the time, does not dispute that he was present when Collier was killed on the evening of April 18, nor does he deny that he participated in the bombings three days earlier that killed three people and hurt more than 240 others.

Prosecutors say Tsarnaev and his older brother, Tamerlan Tsarnaev, killed Collier because they wanted his gun. But their efforts to take it were thwarted by a safety holster.

The FBI had released photos of the pair five hours earlier, and they were on the run. But Tamerlan, 26, would not survive the night. He was killed in a chase and gunbattle with police that began with reports of an "officer down" at MIT.

The MIT police, who are designated as special officers by the Massachusetts State Police, patrol the sprawling campus in Cambridge. Collier's beat was the area of North Quad near Main and Vassar streets. He had been handling a routine call about a citizen who was upset his car had been towed.

A young man called 911 about 10:20 p.m. and reported hearing loud noises outside his window.

Sacco tried to summon Collier on his police radio. No response. He sent an emergency alert. Nothing. He tried texting him. Still no response.

"It became an amount of time that wasn't comfortable," he said.

Sgt. Clarence Henniger had returned to the station at the end of his shift, and had just passed Collier on the way in. He saw nothing out of the ordinary. But when he heard dispatch couldn't raise Collier, he went back out to check on him. His car was in the same spot.

"I parked about 8-10 meters away from Officer Collier's car," Henniger said. "When I arrived at the cruiser I looked inside and that's when I observed a wound to the head, to the temple. I observed a wound to the neck and I observed a wound to his hand."

Prosecutor Weinreb asked: "Was there blood?"

"Yes sir," the witness responded.

"Where?"

"All over the car and his body."

Jurors heard Henniger's frantic call over the police radio:

"Officer down! Officer down! Get me help! Officer down." He called for an ambulance, shouting, "Get on it!!!"

He and another officer pulled Collier out of the squad car to attempt to revive him.

"The amount of blood on his body made it difficult to get a grip on him," Henniger said. The other officer urged Collier to "Hang in there, just hang in there," and asked "Who did this to you?"

Collier did not respond.

Photos:Boston Marathon bombing evidence

Photos:Boston Marathon bombing evidence

A jury condemned Dzhokhar Tsarnaev to death on Friday, May 15, for his role in killing four people and wounding hundreds more in the 2013 Boston Marathon bombings. See photos that were released as evidence in his trial.

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Photos:Boston Marathon bombing evidence

This undated photo of a young Tsarnaev with his brother, Tamerlan, was shown by the defense in the sentencing phase of the trial. Tamerlan died after being shot by police and run over by a car driven by his brother in the massive manhunt that followed the bombings.

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Photos:Boston Marathon bombing evidence

Katie Russell met Tamerlan Tsarnaev at a nightclub and dropped out of college to marry him. Her mother, Judith Russell, testified that Tamerlan came between Katie and her family and that Katie became isolated. She eventually converted to Islam and changed her name to Karima Tsarnaeva. She was the breadwinner. But when company came for dinner, she cooked, served the men and then retired to another room.

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This collection of photos of Dzhokhar Tsarnaev in his wrestling days was introduced by the defense.

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This photo of Tamerlan Tsarnaev, recovered from his computer, was shown during the sentencing phase.

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This image shows victims' positions in the crowd prior to the Boston Marathon bombings on April 15, 2013.

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Tsarnaev "flips the bird" in a jail cell during his first arraignment on July 10, 2013. The image was presented to jurors in the sentencing phase of his trial.

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Tsarnaev poses in front of a black standard adopted by various militant Islamist groups in this Instagram photo that was entered as evidence.

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Prosecutors say Tsarnaev was a self-radicalized jihadist who pored over militant writings, including the article "How to Build a Bomb in the Kitchen of Your Mom." It was found on his laptop and other devices, part of a full-edition download of Inspire magazine, a glossy English-language propaganda tool put out by al Qaeda in the Arabian Peninsula.

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Photos:Boston Marathon bombing evidence

This Russian manual on how to fire a handgun was found in the apartment where Tsarnaev's brother, Tamerlan, lived. Tamerlan Tsarnaev was killed in a shootout with police in Watertown, Massachusetts, on April 19, 2013.

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This copy of The Sovereign, which calls itself the "newspaper of the resistance," was also found in Tamerlan Tsarnaev's apartment.

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Pictured here is a box of bullets found on a street after the shootout in Watertown. The brothers' fingerprints were on the box, prosecutors said.

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A pressure cooker was embedded in the side of a resident's Honda during the Watertown shootout.

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Photos of the Watertown shootout were entered into evidence. Neighbors came to their windows and then retreated. One grabbed his infant son and headed toward the back of his house with his wife. Another grabbed a camera and took photographs from an upstairs window.

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The Tsarnaevs had carjacked a Mercedes SUV in Watertown before the shootout. The vehicle was covered in bulletholes, and the rear window was shattered.

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This unexploded pipe bomb was found at the scene of the shootout between police and the Tsarnaev brothers in Watertown.

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Prosecutors said these boards were attached to the boat where police found Dzhokhar Tsarnaev hiding. A carved message reads, "Stop killing our innocent people and we will stop."

Prosecutors say this surveillance image shows Tsarnaev visiting an ATM hours before a police chase and chaotic shootout in which more than 200 rounds were fired.

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Another view of Tsarnaev's visit to the ATM.

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Exhibits related to the shooting death of MIT Officer Sean Collier were introduced to the jury on Wednesday, March 11. This image from the crime scene appears to show a bloody gun.

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This burned tank top and yellow hoodie belonged to bombing survivor Jessica Kensky.

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Prosecutors say this Fox Racing logo was from one of the backpacks containing a bomb.

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Prosecutors say this still image from surveillance video shows Tsarnaev in the UMass Dartmouth gym the day after the bombings.

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Prosecutors showed the jury photos of what they say are Tsarnaev's writings inside the boat he was captured in.

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This image is from a surveillance camera outside the Forum restaurant in Boston's Copley Square just after the bombing.

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Prosecutors presented two Twitter accounts linked to Tsarnaev that, they said, showed targeting the marathon had been on his mind for at least a year. One account, @J_tsar, contained 1,100 tweets and was the more mainstream of the two. On the day of the 2012 Boston Marathon, a tweet from the account read, "They will spend their money & they will regret it & they will be defeated."

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Prosecutors said the second Twitter account is evidence that Tsarnaev led a double life. By day, he was a slacker college sophomore. By night, he was a wannabe jihadist, posting on the account @Al_firdausiA. In one tweet, he urged people to listen to radical cleric Anwar al-Awlaki's lectures. "You will gain an unbelievable amount of knowledge," he said in March 2013, just weeks before the bombings. Prosecutors also allege in an indictment that Tsarnaev downloaded al-Awlaki's writings, calling him a "well-known al Qaeda propagandist." Al-Awlaki had been killed in a U.S. drone strike in 2011.

A closer view of 8-year-old Martin Richard in the crowd before the bombing.

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Boston police tend to a wounded child. CNN has chosen not to show the young victim's face.

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Mayhem along Boylston Street.

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Cambridge police officer Brendan O'Hearn joined them, and took over applying chest compressions.

"His face and neck were covered with blood. There was some type of wound to his head," O'Hearn said. Collier was gurgling and blood was coming from his mouth

"There was blood everywhere," O'Hearn added.

Weinreb asked, "Did it transfer to you?"

"All over me."

Collier became the fourth victim of the Tsarnaev brothers.

Campus surveillance cameras captured the encounter between Collier and his killer. The footage was shot from a distance and at times, it is difficult to determine whether the cameras captured one person and a shadow or a pair walking closely together.

The video shows Collier's patrol car idling near the front of the Koch Institute building, and a person or two people rounding the corner and walking towards it. The brake lights flash on, then off, and then off again. Two people can be seen running back around the corner.

Off to the side, somebody on a bicycle rides by without stopping.

Harman, the grad student on the bicycle, often used the word "they" in his testimony at first, but said he saw just one person as he pedaled past.

"When I went by, the front door was open, the driver's side door," he said. "There was someone leaning into the driver's side door." He said the person -- although he used the word "they" -- was bent at the waist and leaning into the patrol car.

"They stood up, startled, as I rode my bike by them," he added.

Later in his testimony, he began referring to "they" as "he."

"I only saw one person," he said. "He sort of snapped up, stood up, and turned around. He looked startled. We made eye contact."

Asked to describe who he saw, Harman added, "I remember thinking he had a big nose."

He looked across the courtroom at Tsarnaev, pointed, and said, "He's right there."